ci: require a published snapshot before bumping the moxygen pin

[gmarzot]

The sync pins the newest moxygen main commit whether or not its snapshot exists
yet. moxygen ci-main takes ~80 minutes to build and publish, so a sync landing
inside that window pins a rev nothing can consume, and every moqx lane compiles
the Meta stack from source instead.

Observed

2026-08-28, moxygen 68465b9:

16:15  moxygen ci-main starts building 68465b9
16:21  moqx sync PR opens, pinned to 68465b9      <- 69 min early
17:30  moxygen publishes snapshot-68465b9ffa19

Every lane in that PR fell back to source: linux 49 min, conformance 45/45/37,
macOS 39, asan 34 — roughly four hours of runner time for a one-line pin bump.
Build-step times in the conformance lanes were 2062s, 2590s and 2620s against
286s, 212s and 280s on a normal pin bump.

Change

Scheduled and manual runs walk back from main until they find a rev whose
snapshot-<sha12> release has assets, rather than taking the newest rev
unconditionally.

• Nothing found in 20 commits — skip with a warning and no PR. A publish
  backlog is transient; a red sync every hour is not useful.
• repository_dispatch keeps its payload rev but errors if the assets are
  missing. The publish is what raises that event, so a miss means it reported a
  success it did not deliver.

The cron cascade already tries to sequence this by spacing moxygen's sync at 04:23
against moqx's at 08:23. That only holds when schedules fire on time, and they
have been arriving hours late; the case above was a manual dispatch, which lands
wherever it lands. This makes the ordering a property of the data rather than of
the clock.

Verifying

gh api prints its 404 body on stdout, so the predicate takes the exit code as
the answer and only compares a value that is actually a count — the obvious
|| echo 0 form appends to the error JSON and fails the comparison with
integer expression expected, once per skipped commit.

Checked against live data: the predicate returns true for the current main rev
(14 assets) and false, silently, for a nonexistent one. Selection is a no-op today
because the newest rev already has assets, which is the intended behaviour — it
only diverges inside a publish window.

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This change is